Russian opposition activist Ildar Dadin, only recently freed after spending time in jail for repeated participation in unsanctioned rallies, has been detained again in Red Square, according to a group monitoring protest activity

At least 11 activists were detained on Moscow's Red Square on Friday as they were reading Russia's constitution out loud, a group that monitors protest activity said.

OVD-Info, a website that tracks the detention of activists, said among the activists detained was Ildar Dadin, the first and only person in Russia to have served prison time for contravening a tough law clamping down on protests in the country.

The activists were taken to a nearby police station, OVD-Info said. It remains unclear whether they could face charges.

"A policeman approched us and warned us that if we don't leave, they will detain us," Dadin told TASS news agency. "Ten minutes later police detained us."

Lawyer Alexei Lipster told RIA Novosti news agency that Dadin was still in police custody but that officers had not written up a report.

Dadin, 35, was jailed in December 2015 for the supposed crime of holding repeated peaceful demonstrations against President Vladimir Putin's rule without official permission.

In a letter to his wife smuggled out from prison in November, Dadin alleged he had been tortured behind bars, as well as threatened with rape and murder.

His allegations exploded into the public eye, shining a spotlight on abuse that forced the Kremlin to pay attention.

Putin in January ordered a review of the penitentiary service's work, although it was not explicitly linked to Dadin's case.

In 2014 Russia controversially introduced criminal charges for those who breach rules at protest rallies twice or more in a period of 180 days.